RDS is an abbreviation for Radio Data System. This is a standardized system in Europe for transmitting broadcasting or radio station information to radio receivers. Such radio station information, hereinafter also referred to as RDS information, in coded manner transmits the name or part of the name of the radio station sending the RDS information as well as information on additional frequencies via which the program of this radio station is broadcast. With the aid of the RDS signal it is possible on the one hand to display the name of the station received at the moment on a display of a radio receiver equipped for RDS, and on the other hand it is possible for the radio receiver, when the radio signal of the just received radio station cannot be received any more with sufficient quality on the transmitter receiving frequency set at the particular moment, to automatically switch over to a different transmitter frequency on which the same program of the same station is transmitted.
More details on the RDS system are described in the publication "Specifications of the Radio Data System RDS for VHF/FM Sound Broadcasting", Tech. 3244-E, Technical Centre of the European Broadcasting Union Brussels, March 1984.
The RDS data is modulated onto a carrier frequency of 57 kHz by a biphase modulation. This carrier frequency is not transmitted to the receiver. Due to the fact that RDS information is transmitted in the form of an asynchronous pulse sequence, the carrier frequency of 57 kHz must be produced on the receiver side. Upon synchronous demodulation on the receiver side with the aid of the carrier frequency of 57 kHz produced on the receiver side, the RDS information is present in the form of a digital biphase signal. For obtaining the RDS data on the receiver side, the bit rate clock signal has to be recovered on the receiver side. To this end, a so-called Costas circuit is usually employed.
Each bit of the digital RDS signal that is demodulated on the receiver side is composed of two half bits, with a digital transition from 0 to 1 or from 1 to 0 occurring between the first and second half bits of each RDS bit. The direction in which the transition from the first to the second half bit takes place is dependent on whether the particular RDS bit serves to transmit the logic value "1" or the logic value "0". At the transition between two adjacent RDS bits, a transition from 0 to 1 or from 1 to 0 occurs only when the adjacent RDS bits transmit the same logic value. When adjacent RDS bits transmit different logic values, no transition from 0 to 1 or vice versa will take place between them.
In order to be able to safely recover the RDS information on the receiver side, the digital RDS signal that is demodulated on the receiver side and the bit rate clock signal produced on the receiver side do not only need to have the same bit rate, but they must also be in phase synchronism. This is effected by means of a PLL. Upon turning on a radio receiver or tuning thereof to a different transmitter receiving frequency than that received so far, the PLL needs a certain time for latching to the new transmitter receiving frequency, i.e., after such switching on or over it takes a certain time until phase synchronization is established between the digital RDS signal demodulated on the receiver side and the bit rate clock signal produced on the receiver side. The latching time within which such phase synchronization is established is about 20 ms in practical applications.
The quality of an RDS demodulator depends, among other things, on the latching time. Thus, attempts are made that the period until which phase synchronization is established after switching on or over be made as short as possible.